1. Field of the Invention
The present invention relates to a coating composition capable of being cured by crosslinking by a urethanization reaction of a hydroxyl group and an isocyanate group and being made to a high solid content type having more than 70% by weight of solid content at the time of coating, and relates to a multilayer coating film formation process using the composition.
2. Description of the Prior Art
In the field of coating materials, the reduction of amount of usage of organic solvents is an urgent matter from a viewpoint of environmental conservation against atmospheric pollution and saving of resources etc.
As one of the countermeasures, there is mentioned the development of so-called "high solid content coating materials" in which the amount of usage of organic solvents in the coating materials is reduced to obtain a high solid content concentration in the organic solvent type coating materials.
Most of the high solid content coating materials currently under development contain as resin component a hydroxyl group-containing resin compounded with a melamine resin as a curing agent. Such coating materials, however, have the following disadvantages.
Firstly, it is necessary to make the viscosity of hydroxyl group-containing resin and melamine resin lower in order to achieve a high solid content concentration. However, when the molecular weight of these resins is made lower, there arises a problem that the properties of the coating film are lowered. In order to realize lower molecular weight of resins and sufficient properties of the coating film at the same time, it is necessary to have a higher content of crosslinking functional groups, such as hydroxyl group, methylol group etc. in the resin. However, crosslinking functional groups generally increase the viscosity due to a mutual action among functional groups, and there arises a problem that the lowering of viscosity becomes difficult, if the content of functional groups is higher.
Secondly, in case a high solid content coating material is a coating material using melamine resin as curing agent, a large amount of by-products such as alcohol at the time of curing by heating are generated and there arises a problem that popping caused by baking happens in the coating film quite often.
Moreover, in the top coating film of automobile outer plates etc. to be used outdoors, etching and stains caused by the acid rain are worldwide problems. Another problem is scratches caused by a car-washing machine as the machine has been popularized. Therefore, there has been demanded a development of a coating material for top coat, especially a clear coating for top coat which can form a coating film having both enough acid resistance against acid rain etc. and scratch resistance. The coating film obtained from the coating material containing resins of a combination of the above-mentioned hydroxyl group-containing acrylic resin and a melamine resin has a problem of inferior acid resistance and scratch resistance.
Urethane type coating materials, which are crosslinked and cured by a urethanization reaction between a hydroxyl group and an isocyanate group, can form a cured coating film excellent in acid resistance, scratch resistance, coating appearance (for example, gloss, fatness, image sharpness) etc. and are widely used. However, it is difficult for the urethane type coating materials to bring the solid content at the time of coating to a higher solid content concentration of more than 70% by weight, without deteriorating these properties and therefore their applications are limited in recent years where prevention of atmospheric pollution and saving resources are seen as important, without being able to sufficiently utilize the above-mentioned advantages.
The purpose of the present invention is to propose a urethane type coating composition capable of being made the solid content at the time of coating to more than 70% by weight, without deteriorating good properties, which a urethane type coating material has, such as acid resistance, scratch resistance, finishing appearance etc. and a multilayer coating film formation process using the coating composition.
As a result of an extensive study, the present inventors have found that the above-mentioned purpose can be achieved by a coating material comprising a vinyl type polymer having both silylated hydroxyl groups and alkoxysilyl groups, low molecular weight polyol, polyisocyanate compounds and a silanol group forming catalyst, and optionally containing a small amount of an organotin compound, and completed the present invention.